1. Field of the Invention
The present invention relates to an integrated AFM sensor used in an atomic force microscope.
2. Description of the Related Art
As an apparatus capable of observing a conductive sample in a resolving power of an atomic order, an STM (Scanning Tunneling Microscope) was devised by Binnig and Rohrer et al. In this STM, a sample which can be observed is limited to a conductive one. For this reason, an AFM (Atomic Force Microscope) is proposed as an apparatus which can observe an insulating sample at a resolving power of an atomic order by using STM element techniques such as a servo technique. This AFM is disclosed in U.S. Pat. No. 4,724,318.
The AFM includes a cantilever having a sharp projection portion (probe) at its free end. When the probe is brought close to a sample, the free end of the cantilever is displaced by interaction (interatomic force) acting between atoms of the distal end of the probe and atoms of the surface of the sample. While the displacement of the free end is electrically or optically measured, when the probe scans the sample along the surface of the sample, three-dimensional information about the sample can be obtained. For example, when the probe scans the sample while the distance between the probe and the sample is controlled to keep the displacement of the free end constant, the distal end of the probe moves along the uneven surface of the sample. Therefore, a three-dimensional image representing the surface shape of the sample can be obtained from the positional information of the distal end of the probe.
In the AFM, a displacement measuring sensor for measuring the displacement of a cantilever is generally arranged independently of the cantilever. In recent years, an integrated AFM sensor in which a function capable of measuring the displacement is added to a cantilever itself is proposed by M. Tortonese et al. The integrated AFM sensor is disclosed in, e.g., "M. Tortonese, H. Yanada, R. C. Barrett and C. F. Quate, Transducers and Sensors' 91: Atomic Force Microscopy Using a Piezoresistive Cantilever".
Since the integrated AFM sensor has a very simple and small structure, the AFM sensor is expected to constitute a so-called stand-alone AFM having a movable cantilever. In a conventional AFM, since a relative positional relationship between the probe at the distal end of the cantilever and the sample is changed by causing the sample to move in X and Y directions, the maximum size of the sample is limited to about several centimeters. The stand-alone AFM can be advantageously free from the limit of the sample size.
In the integrated AFM sensor, the probe is not particularly arranged at the free end of the cantilever, and the distal end of the cantilever is formed into the shape of a triangle, though it has a planar structure, and the triangular end is brought close to a target sample, thereby performing AFM measurement. However, in this measurement, a high resolving power in the direction of the sample surface cannot be expected.
In addition, in a method of manufacturing the integrated AFM sensor, a support portion for holding a cantilever is formed by one silicon wafer, and the thickness, length, and the like of each part cannot be standardized at high accuracy. More specifically, the thicknesses of commercially available 4-inch-diameter silicon wafers which are popularly used have large variations, i.e., about 525 .mu.m .+-.20 .mu.m. In integrated AFM sensors, since silicon wafers are etched to form cantilevers, the lengths of the cantilevers have variations depending on the variations in thicknesses of the silicon wafers. As a result, sensors each having a high resolving power cannot be provided.
In recent years, in a normal AFM, during AFM measurement, a tunnel current flowing between a sample and a probe is monitored while the AFM measurement is performed, thereby simultaneously performing AFM/STM measurement. In addition, a capacitance between the probe and the sample is detected to perform AFM/SCaM (Scanning Capacitance Microscopy) measurement. However, in an integrated AFM sensor, since two or more types of signals cannot be captured at a high resolving power, the AFM/STM simultaneous measurement and the AFM/SCaM measurement cannot be performed.
In an AFM, the distal end of the cantilever is brought maximally close to the surface of the sample to perform scanning, and the atoms of the sample surface cause a repulsive force and an attractive force to act on the atoms of the distal end of the cantilever. Since the 10 repulsive force and attractive force acting on the atoms act in the surface normal direction of the uneven portion of the sample surface and are changed every scanning moment, non-uniform torsion occurs in the cantilever. The image of the sample surface obtained on the basis of the warpage of the cantilever is caused to be inaccurate by the torsion in the cantilever. For this reason, the following attempt is performed in the normal AFM. That is, the torsion of the cantilever is detected and corrected, thereby obtaining a more accurate image of a sample. However, the integrated AFM sensor does not have a function of detecting this torsion of the cantilever.